A preliminary study of some "pseudo-AB₂" compounds: RENi₄ Mg with RE = La, Ce and Gd. Structural and hydrogen sorption properties

The compounds RENi₄ Mg have been elaborated by mechanical alloying (MA) and a subsequent heat treatment at 600 ^{{ring operator}} C for 1 h. The lattice parameters are in good agreement with those already reported: 7.165 over(A, ̊) for LaNi₄ Mg, 7.030 Å for CeNi₄ Mg and 7.038 over(A, ̊) for GdNi₄ Mg. The hydrogen sorption behaviors are strongly influenced by the rare earth used: for La compound, hydrogen absorption leads to a structural change and no desorption can be observed, for Ce compound, no reaction towards hydrogen can be noticed and for Gd compound, hydrogen can be absorbed and desorbed reversibly at room temperature. Moreover, for GdNi₄ Mg, no structural change is reported after hydrogenation and only a slight volume expansion can be noticed (e.g. 5%). The MA process has also been used successfully for the elaboration of GdNi_{4 - x} Al_x Mg up to x = 1. In these substituted compounds, hydrogen can also be absorbed and desorbed reversibly at room temperature.